1. Field of the Invention
The invention relates generally to an apparatus and method for acquiring satellite signals and more particularly, to an apparatus and method for acquiring GPS signals that combines parallel and sequential detection processes.
2. Description of the Related Art
The requirements for fast acquisition in weak signal and high jamming environments has led the GPS industry to massively parallel architectures for both commercial and military applications. These massively parallel architectures comprise hundreds of thousands of correlators and dozens of FFT taps. This architecture allows searches of large regions of the time-frequency uncertainty domain in a single dwell of the acquisition engine. However, since all elements of the parallel architecture are treated equally, it has not been possible to realize the advantages of a sequential detector in minimizing the dwell time as a function of the dynamically observed signal and noise environment.
Typical GPS signal acquisition processes comprise two steps. In the first step of the process, the largest signal detected in an array of thousands of time/frequency cells is compared against a threshold. In the second step of the process, if the largest detected signal exceeds the threshold, a verification/false alarm rejection algorithm attempts to detect the signal again, multiple times to verify that the signal does indeed exceed the software threshold.
In the first step, the architecture of the GPS receiver, specifically the acquisition correlators, form a correlation output over the desired coherent and non-coherent integration period and report the time and frequency coordinates of the peak detected signal that exceeds a software-controlled threshold. Once these coordinates are reported, the architecture uses the same acquisition correlators previously used to detect the peak signal to attempt to detect the signal in multiple, repeated dwells to confirm the presence of the signal.
One drawback with this technique is the large sensitivity to the selected threshold. If the threshold is too high there is a large probability of a missed detection, if it is too low the search rate will be slowed due to false alarms. Another significant drawback to this GPS acquisition process is that it requires the reported signal to be the largest detected out of thousands of signals. Assuming that the desired probability of acquisition is 98%, this means that the probability that a noise sample exceeds the signal plus noise must not exceed a 2% missed detection probability. For a 511 correlator, 64-tap FFT architecture having 32,704 cells, this requires a signal-to-noise (S/N) ratio of 6.95. Achieving such a S/N ratio implies either a lower jam immunity/weak signal sensitivity, or an increase in the time-to-first-fix (TTFF) while the signal is integrated to the required S/N.
Hence, those skilled in the art have recognized the need for a GPS signal acquisition process having signal detection capability that is not so heavily dependent on the setting of a threshold level, as are current processes. The need has also been recognized for a process that retains the wideband search capabilities of existing massively parallel architectures, but also includes many of the advantages of a sequential detector.